Unless & If

Lexical Level	Meaning	Sentence
Unless	Except under the condition that if ... not	I will not help you unless you ask me. = I will not help you if you do not ask me.
If	In the event that	I will help you if you ask me. = I will help you in the event that you ask me.

To change from ‘if’ to ‘unless’ 1) Always change the ACTION to the ‘opposite’ meaning. 2) Do not alter the CONDITION. 3) Use if...not to check if the change is correct.
ACTION Second event	Conj	CONDITION First event
+ve		+ve
I will meet him	if	I have the time.
Change +ve  to –ve.	No change.
I will not meet him	unless	I have the time.
–ve		+ve
I will not play soccer	if	it rains.
Change -ve  to +ve	No change.
I will play soccer	unless	it rains.

  
    Check:  I will not meet him    if                 I do not have the time.
                 I will play soccer      if                 it does not rain.